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Effect of Heat Stress On Nonreturn Rate in Holstein Cows: Genetic Analyses

Department of Animal and Dairy Science, University of Georgia, Athens 30602

Corresponding author:
I. Misztal; e-mail:
ignacy{at}uga.edu.

The genetic component in heat tolerance for nonreturn rate in Holsteins was estimated using an animal linear model augmented by a random regression on a temperature-humidity index (THI). Data consisted of 18,059 nonreturn rates at 45, 60, and 90 d after insemination and 81,674 first-parity test-day milk yields from 78 herds in Florida. The THI on the day of insemination or test day was added to each record. Only first-insemination records were used. The model for nonreturn rate included the effects of herd-year-season, age, days in milk, milk yield, THI as a covariable, regular additive effect, and random regression on THI for heat-tolerance additive effect. With a single-trait model, heritability estimates for NR45, NR60, and NR90 at THI = 70 for first-lactation cows were 0.006, 0.014, and 0.053, respectively. Genetic correlation between regular NR90 and heat tolerance was –0.95. A bivariate analysis for NR90 and test-day milk production yielded a correlation between regular merit and heat tolerance for NR90 of –0.35, substantially lower than by the univariate model, indicating a bias in the univariate estimates caused by ignored selection. The regular genetic correlation between NR90 and milk yield was –0.41. Genetic correlation between heat tolerance for NR90 and heat tolerance for milk yield was –0.04, indicating the need to separate selection.

Key Words: heat stress • dairy cattle • nonreturn rate • temperature-humidity index

Abbreviation key: NR = nonreturn rate, THI = temperature-humidity index

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